In optical measuring systems that provide for spectral measurements, i.e. measurement of optical properties as a function of optical wavelength or at one or more selected wavelengths, a wavelength calibration may be necessary. One example of such an optical measuring system uses a fibre Bragg based sensor device. Such a sensor device reflects light in the fibre, with wavelength dependent reflection properties that vary with external temperature. In order to use the fibre Bragg grating to measure physical parameters such as stress or absolute temperature, a measurement of the wavelength of the reflected light is needed.
One solution is the use of a tunable narrow band reference light source (a laser) and detection of the response as a function of tuning. However, the tuning range of such light source is often limited and widening the tuning range significantly increases costs. Moreover, the measurements can be disturbed if the light source suffers from mode hops. Another solution is to use broadband light and detection of the response using a monochromator. But the use of a monochromator significantly increases the cost of using simple sensor devices like fibre Bragg gratings. A monochromator is large and not robust to handling. A lower cost solution is the use of broadband light with a low cost tunable narrow band filter in the light path.
US Patent application 2004091002 discloses a method of tuning a filter wherein the resonance frequency of the filter is calibrated by means of two Fabry-Perot interferometers. Such interferometers act as comb filters that pass light only at a series of discrete wavelengths. In US2004091002 a first Fabry-Perot interferometer is dimensioned to provide relatively small differences between successive wavelengths of the comb filter and a second Fabry-Perot interferometer is dimensioned to provide larger differences between wavelengths of its comb filter. The second Fabry-Perot interferometer is combined with a filter to block light from all but one of these wavelengths. In US2004091002 the second Fabry-Perot interferometer is used to provide an absolute wavelength reference and the first Fabry-Perot interferometer is used to count the number of peaks of between the peak of the second Fabry-Perot interferometer and the actual frequency of the tuned filter.
In operation, light from a tunable filter (e.g. a laser cavity) is passed to the two Fabry-Perot interferometers, the resonance wavelength of the tunable filter is scanned. From the outputs of the Fabry-Perot interferometers it can be determined when the tunable filter is tuned to the second Fabry-Perot interferometer and how many peaks of the first Fabry-Perot interferometer have passed between tuning to the current resonance wavelength and tuning to the second Fabry-Perot interferometer. From this the current wavelength is determined.
U.S. Pat. No. 5,892,582 discloses a similar system, but with a fibre Bragg grating to perform the function of the combination of the second interferometer and the filter that blocks all but one wavelength. Both US2004091002 and U.S. Pat. No. 5,892,582 provide for solutions to determine wavelengths that can be implemented at low cost, or even integrated in a small optical device. It can be used to determine the wavelength of a laser that is tuned by means of a tunable filter. When the laser is tuned to a peak of the first Fabry Perot interferometer the wavelengths is known exactly. When the laser is tuned between peaks the wavelength can be estimated by interpolation, after measuring the amount of tuning needed to pass from one peak of the first Fabry Perot interferometer to the next.
However, this type of wavelength determination suffers from the problem that it cannot measure wavelengths when laser is used that suffers from mode hopping. Unfortunately many low cost lasers suffer from mode hopping, or start suffering from mode hops due to ageing. Mode hopping can have the effect that interpolation of the wavelength becomes useless. When mode hops across peaks of the first Fabry Perot interferometer occur, the count of peaks from the peak of the second Fabry Perot interferometer may even become erroneous.